It would be pointless to upgrade to a Core 2 Duo, so just shop around online for the fastest Core 2 Quad you can find for a reasonable price. You also want 2GB more RAM.
The sluggishness is probably the low amount of RAM, or the relatively slow hard drive. If you have Windows 7, I highly recommend buying a Crucial Adrenaline or Mushkin Catalyst for a nice speed boost in pretty much everything.
if i would just upgrade my motherboard,cpu, and ram cause the prices for fast core 2 quads are basically around the same price of a 2gen/3gen low i5's that will be still way faster then them core 2 quads and in one more year you'll be hear looking to upgrade that. thats not all though ddr2 is 2 times the price of ddr3 so just get a new motherboard for like around 50$ ivy bridge celeron-40$ and 4gb ddr3-20-30$ and and if you need more power get upgrade your cpu to a i5 but since you say you dont game it wont be necessary so you can stop at an i3